1. Field of the Invention
The invention relates to a process for purifying exhaust gases containing nitrogen oxides, in particular an exhaust gas of a combustion engine. The invention further relates to a device for carrying out such a process.
Because of the formation of ozone from nitrogen oxides which takes place in bottom air layers predominantly in summer, there is a compelling requirement for effectively removing nitrogen oxides from the exhaust gases of combustion engines. For spark-ignition engines running with an excess air factor =1, three-way catalysts remove, in addition to nitrogen oxides, uncombusted hydrocarbons and carbon monoxide as well. For diesel engines and lean-burn engines (i.e. spark-ignition engines working with an excess air factor &gt;1), these catalysts cannot be used on account of the high oxygen content in the exhaust gas and a type of catalyst which can be used for such exhaust gases with satisfactory pollution abatement rate and useful life has not yet been found.
With previously published DE A 42 31 581 and WO A 94/6676, plasma processes for purifying such exhaust gases with high oxygen content have already been disclosed. Furthermore, German Patent Applications 195 25 749.9 and 195 25 545.0, which were not yet published at the priority date of the present application, have also disclosed combinations of gas discharge and catalytically active wall materials for achieving good energy efficiency in breaking down pollutants. This combination might also be sensible because new discoveries have given reason to suspect that, in silent discharges or other suitable plasma processes, nitrogen oxides are predominantly oxidized to form HNO.sub.2 and HNO.sub.3 instead of being reduced to the desired products N.sub.2 and O.sub.2. There has incidentally been research into plasma-induced catalysis, but this gives little hope of catalytic materials which, in combination with gas discharges, promote the reduction of nitrogen oxides to form N.sub.2 and O.sub.2 to such a great extent that the oxidation to form acids can be neglected. Either the catalytic material also promotes oxidation, or oxidative adsorption takes place. During oxidative adsorption, the surface of the catalyst soon becomes saturated. This means that such a catalyst has, for example in a steady-state system, to be regenerated by desorption and the pollutants then released have to be neutralized. This problem can be avoided according to the prior art only if a reducing agent such as, for example, fuel, is added to the exhaust gas in a high proportion. This in turn increases the fuel consumption significantly and causes problems with the emission of incompletely reacted reducing agents.
In other research, it has been found that it is possible to achieve high degrees of breakdown for nitrogen oxides by adding ammonia as a reducing agent to the exhaust gas before the treatment in the discharge. However, this entails the following problems: first, a motor vehicle equipped with such an exhaust-gas purification system must carry ammonia or a less harmful related substance from which ammonia can be generated; second, the addition of this far from harmless reducing agent must be very accurately controlled, in order to avoid leakage, and third, the resulting ammonium nitrate dust must be filtered from the exhaust gas. Because of these problems, this solution is not currently acceptable to the automobile industry. There has to date been no other reducing agent in liquid or gaseous form with which reduction even approximately as efficient as with ammonia can be achieved. However, considerably more promising for purifying diesel exhaust gases has been research with activated carbon fibers, possibly doped with catalytically active metals, the fibers functioning here as a solid reducing agent: NO.sub.x decomposition degrees of up to 70% have thereby been achieved. However, this presupposes correct pretreatment of the fibers and an exhaust-gas temperature which is neither too high nor too low. If the exhaust-gas temperature is too low, as in cold starting, engine idling or under low load, the degree of decomposition is low, whereas at too high an exhaust-gas temperature, accelerated oxidation of the fibers by oxygen in the exhaust gas, i.e. erosion of the fibers, takes place. On account of the thermal inertia of typical exhaust-gas systems in motor vehicles it is scarcely possible, and not even sensible in energy terms, to control the temperature of the exhaust-gas cleaning element actively. There is thus a need for a way of reducing nitrogen oxides using carbon fibers other than by controlling the temperature.
EP 0 621 263 A1 discloses a process for reducing nitrogen oxides in diesel exhaust gas using activated carbon, which is combined with an electrostatic soot-filtering process. In order to filter the soot, the soot particles are charged by applying an electric voltage so that they can subsequently be deposited electrostatically. Merely by virtue of the way in which the electrostatic soot filter operates, it does not have a direct effect on the nitrogen oxide reduction. One indirect effect could consist in the soot collected in the filter itself acting as a solid reducing agent.